ULA lobbied to ban them from a launch site (at Vandenberg) that was nearby their own, which had a very expensive rocket and very, very expensive payload on it, claiming that a SpaceX rocket launch would be too dangerous. Forced them to launch from Kwajalein Atoll, which made things much harder. Probably contributed to at least one failure, thanks to the corrosive, salty atmosphere.

Edit: Not unfair to assume that it was motivated by anti-competitive interests before any real concern about possible effects on their pad.

What was the payload that was so expensive?

A large NRO spy sat of some sort.